1. Field of the Invention
The present invention relates to an additional information embedding method, additional information detecting method, additional information embedding apparatus and additional information detecting apparatus which are utilized to embed plural pieces of additional information, for example, on a video signal as electronic watermark information, and to detect the plural pieces of additional information from the video signal.
2. Description of the Related Art
Increasingly spreading digital contents such as the Internet, compact discs (CD), digital video discs (DVD) have given rise to a problem of copyright infringement due to unauthorized copies of such digital contents. To solve this problem, a copy control scheme has been contemplated, where additional information for copy control (hereinafter referred to as the “copy control information”) is embedded on digital contents by electronic watermark processing to prevent unauthorized copies.
The electronic watermark processing adds information as noise, to a portion existing in a video signal or an audio signal which is not important from a viewpoint of human's perception, i.e., to a portion which is not redundant for music or video. Such information added to a video signal or an audio signal by the electronic watermark processing is hard to remove from the video signal or the audio signal. On the other hand, even if a video signal or an audio signal has been filtered or data-compressed, additional information indicative of an electronic watermark (electronic watermark information) added thereto can be detected from the filtered or compressed video signal or audio signal.
With the foregoing copy control scheme utilizing the watermark processing, added additional information may represent four conditions:                (1) “Copy Free”;        (2) “Copy Once”;        (3) “No More Copy”; and        (4) “Never Copy”so as to set forth the copy generation and copy limiting conditions for video signals and audio signals on which the electronic watermark information is embedded.        
(1) “Copy Free” indicates that audio signals and video signals may be freely copied. (2) “Copy Once” indicates that audio signals and video signals may be copied only once. (3) “No More Copy” indicates that audio signals and video signals are not allowed to be copied any more because the audio signals and video signals which might be copied once already (in the (2) state). (4) “Never Copy” indicates that copies are absolutely prohibited.
With such electronic watermark information indicative of a copy generation and copy limiting conditions added to a video signal or an audio signal, when the video signal or the audio signal is to be copied, the electronic watermark information added to the video signal or the audio signal to be copied is detected in a recording apparatus. Then, the recording may be controlled in accordance with the copy generation and copy limiting conditions indicated by the detected electronic watermark information to ensure that content information such as video signals and audio signals are protected from unauthorized copies.
Recently, however, there is an increasing request for embedding, as electronic watermark information, a variety of further additional information (additional data) such as copyright information, in addition to the copy control information, on content information.
As an example of additional data, if copyright information to specify a copyright owner is embedded on content information as electronic watermark information, this may be detected as required to identify the copyright owner. Therefore, even if the content information on which the copyright information is embedded as electronic watermark information is illegally copied, the copyright information added as electronic watermark information may be detected from the illegally copied content information to identify the legal copyright owner, thereby making it possible to facilitate the exposure of unauthorized copies.
As mentioned above, the additional data is not such highly significant information that is always required upon copy, like the copy control information, but is relatively less significant information which is sufficient if it can be detected as required.
In the case where plural pieces of additional information having different significance degrees, like copy control information and additional data, are embedded on content information as electronic watermark information, the electronic watermark information indicating the more significant additional information, for example, the electronic watermark information indicating the copy control information may be detected with difficulties. In this case, the difficulties in detection would cause an additional problem of a failure in reliable and rapid copy control.